1. The Field of the Invention
The present invention relates to a process and a composition suitable for the forming of complex shapes from ceramic or metallic particulates. More particularly, the present invention relates, among other things, to a well-dispersed, cryoprotected composition which can be molded by freezing without inducing defects or textures, lyophilized without the formation of a continuous liquid phase, and subsequently sintered to form a complex high performance shape.
2. The State of the Art
Complexly shaped, three-dimensional, high performance ceramic parts are essential structural and electronic components for a wide variety of industries. High performance properties are those which closely approach the intrinsic properties of the starting material, for example, strength, toughness, uniformity, surface finish, resistivity, optical properties, and thermal expansion. These and other properties are markedly affected by the quality of the starting materials and the manner in which they are processed. Factors that have limited the production of advanced ceramics for high performance applications are (i) poor strength and reliability, stemming from poor raw material quality and improper processing techniques, and (ii) high cost, stemming from low product yields, long processing cycle times, and high capital equipment expenditure. For example, a high strength, high performance alumina article is one which can in some cases be characterized by a fully densified body homogeneously composed of uniform submicron alumina grains. If a processing step introduces a texture or a defect of a critical size, a strength limiting flaw will have been created and will result in a severe departure from the intrinsic or high performance properties desired.
Historically, ceramics have not been used for high performance applications due to poor starting materials and the incorporation of property limiting defects through inadequate processing, both as mentioned above. Only recently has the ceramics community recognized the importance of both the starting materials and the processing technique on the properties of the article produced.
In general, three-dimensional complexly shaped sintered ceramic and metallic parts are manufactured by thermoplastic injection molding, in which ceramic or metal powders are compounded with a mixture of thermoplastic resins at high torque and at high temperature. The resulting mixture has a dough-like consistency, which is why the compounding process is generally referred to as "kneading." Particle dispersion is difficult and tedious to obtain in such a system, and traditionally has been a source of microstructural defects, such as holes and non-uniform particle packing. The mixture is then fed into a high pressure injection molding machine, usually in the form of granules or pellets. The molding machine and the molds used are typically large and expensive because injection pressures can range from approximately 2500 psi to 30,000 psi, thus requiring mold clamping forces in the "tens of tons" range. As the pellets are fed into the injection molding machine, they are remelted and injected through a sprue into a mold cavity. The high viscosity and dough-like consistency of the molding composition can result in weld or knit lines, mold gate, sprue, and parting line textures, all of which can create property limiting defects. After the part is molded, the thermoplastic/ceramic composition is subjected to binder removal, which is typically a long (requiring days), expensive, and deleterious process, particularly for a fine particle matrix typical of a high performance ceramic body. Initially, binder removal can result in bubble formation, delamination, and blistering of the part. During binder removal, which is commonly practiced by heating the article, the polymer/ceramic composite is heated beyond the polymer softening point, and thus dimensional tolerance is difficult to control. After binder removal, the porous particulate body is sintered at high temperatures so that the particulate structure can fuse together, thereby resulting in a dense, strong ceramic that is approximately 20% smaller than the presintered (green) particulate part. Final machining is generally required due to poor dimensional tolerances, parting lines, and gate remnants remaining on the fired part; this final machining commonly imparts defects to the fired part, thereby creating property limiting, especially strength limiting, defects.
An alternate approach to thermoplastic resin molding has been to substitute low temperature melting, low viscosity waxes in place of the thermoplastic resins. While this substitution allows for low pressure injection molding, the problems associated with dispersion, binder removal, machining, green strength, and dimensional tolerances have kept this particular system from wide commercial acceptance.
Historically, investigators have recognized the limitations that the binder has placed on the processing of complexly shaped, three-dimensional parts. The art later began to understand and appreciate that the binder, which had allowed the ceramic and metal particles to be formed into a shape and later handled, was also the cause of many economic and performance problems. Rivers, U.S. Pat. No. 4,113,480, developed an aqueous-based injection molding process exclusively for metal powders using 1.5 to 3.5 wt. % (metal powder basis) of high viscosity methylcellulose additive to provide green strength. The resulting mixture of metal powder, water, and methylcellulose has a "plastic mass" consistency and could be injection molded at 8,000 psi. The molded mass was then thermally dried and the green part was conventionally sintered. Although binder burnout is eliminated in this particular process, the presence of defects and costs associated with dispersion and molding of a high viscosity mix, as well as the implementation of a necessary but difficult thermal drying step, still remain.
The use of a molding vehicle which could be frozen has been investigated as an alternate method for casting or molding without the use of thermoplastic carriers. Sublimative drying by freeze drying (lyophilization) has been shown to be generally less destructive to the particle fabric in the green part during drying. A. Kwiatkowski et al., "Preparation of Corundum and Steatite Ceramics by the Freeze Drying Method," Ceramurgia International, vol. 6, no. 2, pp. 79-82 (1980). Such a method has been described by Nesbit, U.S. Pat. No. 2,765,512, which describes casting a ceramic shape from a thick slip containing water and ceramic particles which are then frozen into a shape while in the mold. The resulting frozen part was demolded, dried at room temperature and pressure, and subsequently fired. Downing et al., U.S. Pat. No. 3,885,005, has cast coarse grained refractory shapes from a slip containing 70% coarser than #200 mesh ceramic particles, water, and a silica sol binder. The resulting cast shape was subsequently frozen, causing the silica to gel and cementing the refractory particles together. The residual water was frozen and the part was demolded and heated to 200.degree. F. (93.3.degree. C.) to thaw and dry the part. Dennery et al., U.S. Pat. No. 3,567,520, in making metal parts from powdered metals, formed a thin aqueous-based paste sheet into a part, the part was frozen at -60.degree. F. (-51.1.degree. C.) and then freeze-dried to overcome thermal drying stresses which would be destructive to the part. Maxwell et al., U.S. Pat. No. 2,893,102, cast and molded thicker parts from an aqueous ceramic slip in which the slip and mold were frozen in a CO.sub.2 bath followed by freeze drying and sintering.
As a slight departure from the art thus described, Weaver et al., U.S. Pat. No. 4,341,725, describes the use of a cryoprotectant as an additive in an aqueous suspension to inhibit ice crystal growth, which, after drying, can cause severe strength limiting defects. Weaver claims that the foregoing prior art would result in "low performance" articles riddled with scars resulting from ice crystal formation. By using hydrogen bonding additives in a hydrogen bonding medium, Weaver et al. claimed to limit the size of ice crystals formed to those on the order of 20-50 microns (micrometers).
Another technology that has been investigated uses a gelation process to form the green article. For example, Fanelli et al., in U.S. Pat. No. 4,734,237, use an amount of agar in their slurry and effect gelation by raising the temperature. Gelation can also be caused by phase changes, such as freezing, as exemplified by the Blasch et al. disclosure in U.S. Pat. No. 4,552,800, which teaches using a freeze-sensitive colloidal silica sol that-gels irreversibly upon freezing. Yet another method is found in Japanese laid-open application 61-158403 (from application 59-279176), which teaches what is termed freezing the dispersant by lowering the slurry temperature; the final temperature is significantly above the freezing temperature of the slip vehicle. Among the drawbacks to using a gelation process are (i) the continued presence of thermal drying (i.e., that there is a continuous liquid phase during drying, which leads to destructive capillary forces) and (ii) the relatively slow kinetics of gelation in comparison with freezing by the present invention (the slower the solidification rate, the higher the probability for particle rearrangement and degraded green and sintered properties).